The invention relates to prepregs and composite components (mouldings) produced therefrom at a low temperature, obtainable by use of powdery highly reactive polyurethane compositions containing uretdione groups, with specific catalysts.
Various moulding processes, such as for example the reaction transfer moulding (RTM) process, comprise the introduction of the reinforcing fibres into a mould, the closing of the mould, the introduction of the crosslinkable resin formulation into the mould and the subsequent crosslinking of the resin, typically by application of heat.
One of the limitations of such a process is the relative difficulty of laying the reinforcing fibres in the mould. The individual layers of the fabric or non-woven must be cut to size and adapted to the different mould geometries. This can be both time-consuming and complicated, in particular when the mouldings are also intended to contain foam or other cores. Mouldable fibre reinforcements with simple handling and pre-existing reshaping possibilities would be desirable here.
Fibre reinforced materials in the form of prepregs are already used in many industrial applications because of their ease of handling and the increased efficiency during processing in comparison to the alternative wet lay-up technology.
Industrial users of such systems, as well as faster cycle times and higher storage stabilities even at room temperature, also demand the possibility of cutting the prepregs to size, without the cutting tools becoming contaminated with the often sticky matrix material during automated cutting to size and lay-up of the individual prepreg layers.
As well as polyesters, vinyl esters and epoxy systems, there are a range of specialized resins in the field of the crosslinking matrix systems. These also include polyurethane resins, which because of their toughness, damage tolerance and strength are used in particular for the production of composite profiles by pultrusion processes. The toxicity of the isocyanates used is often mentioned as a disadvantage.
Polyurethane composites also exhibit superior toughness compared to vinyl esters, unsaturated polyester resins (UPR) or UPR-urethane hybrid resins.
Prepregs and composites produced therefrom on the basis of epoxy systems are for example described in WO 98/50211, U.S. Pat. No. 4,992,228, U.S. Pat. No. 5,080,857, U.S. Pat. No. 5,427,725, GB 2007676, GB 2182074, EP 309 221, EP 297 674, WO 89/04335, U.S. Pat. No. 5,532,296 and U.S. Pat. No. 4,377,657, U.S. Pat. No. 4,757,120.
In WO 2006/043019, a process for the production of prepregs on the basis of epoxy resin polyurethane powders is described.
Furthermore, prepregs based on thermoplastics in powder form as the matrix are known.
In US 2004/0231598, a method is described wherein the particles are passed through a special acceleration chamber with electrostatic charging. This device is used for the coating of glass, aramid or carbon fibre substrates for the production of prepregs from thermoplastic resins. As resins, polyethylene (PE), polypropylene (PP), polyether ether ketone (PEEK), polyether sulphone (PES), polyphenyl sulphone (PPS), polyimide (PI), polyamide (PA), polycarbonate (PC), polyethylene terephthalate (PET), polyurethane (PU), polyester and fluoro polymers are mentioned. The thermoplastic prepreg textiles produced therefrom exhibit inherent toughness, good viscoelastic damping behaviour, unlimited storage life, and good chemicals resistance and recyclability.
In WO 98/31535, a method for powder impregnation is described, wherein the glass or carbon fibre strands to be impregnated are impacted with a particle/liquid or particle/gas mixture in a defined velocity profile. In this, the powders consist of ceramic or thermoplastic materials, inter alia thermoplastic polyurethane.
In WO 99/64216, prepregs and composites and a method for the production thereof are described, wherein emulsions with polymer particles so small that individual fibre coating is enabled are used. The polymers of the particles have a viscosity of at least 5000 centipoises and are either thermoplastics or crosslinking polyurethane polymers.
In EP 0590702, powder impregnations for the production of prepregs are described, wherein the powder consists of a mixture of a thermoplastic and a reactive monomer or prepolymers.
WO 2005/091715 likewise describes the use of thermoplastics for the production of prepregs.
Michaeli et al. describe the development of a powder technology for a pultrusion process with thermoplastic polyurethanes, referred to as TPU, in Coatings & Composite Materials, No. 19, p 37-39, 1997.
Further, in the article Processing and properties of thermoplastic polyurethane prepreg. (Ma, C. C. M.; Chiang, C. L. Annual Technical Conference—Society of Plastics Engineers (1991), 49th 2065-9.) thermoplastic polyurethane (TPU) prepregs based on TPU systems containing solvents and water are disclosed.
Prepregs with a matrix based on 2-component polyurethanes (2-C PUR) are known.
The category of the 2-C PUR essentially comprises the standard reactive polyurethane resin systems. In principle, this is a system made up of two separate components. While the critical ingredient of one component is always a polyisocyanate, in the case of the second this is polyols, or with recent developments also amino- or amine-polyol mixtures. The two parts are only mixed together shortly before processing. Thereafter the chemical curing takes place by polyaddition with formation of a network of polyurethane or polyurea.
After mixing of the two components, 2-component systems have a limited processing time (stand time, pot life), as the reaction that sets in leads to a gradual viscosity increase and finally to gelling of the system. However, many factors determine the effective duration of its processability: reactivity of the reaction partners, catalysis, concentration, solubility, moisture content, NCO/OH ratio and ambient temperature are the most important [Lackharze, Stoye/Freitag, Hauser-Verlag 1996, pages 210/212].
The disadvantage of the prepregs based on such 2-C PUR systems is that only a short time is available for the processing of the prepreg into a composite. Consequently such prepregs are not stable over several hours, let alone days.
Below there follows a description of the polyurethane prepregs or composites based on 2-C PUR systems.
In the article by K. Recker, the development of a 2-C polyurethane system for the resin mat process with particular reference to the processing properties for SMC components is reported. (Baypreg—a novel POLYURETHANE material for the resin mat process, Recker, Klaus, Kunststoffe-Plastics 8, 1981).
WO 2005/049301 discloses a catalytically activated 2-C PUR system, wherein the poly-isocyanate component and the polyol are mixed and processed into a composite by pultrusion.
In WO 2005/106155, fibre reinforced composites for the construction industry are disclosed, which are produced by the long fibre injection (LFI) technology with 2-C polyurethane systems.
In JP 2004196851, composites are described which are produced from carbon fibres and organic fibres, such as for example hemp, with the use of a matrix of 2-C PUR based on polymeric methylenediphenyl diisocyanate (MDI) and specific OH group-containing compounds.
EP 1 319 503 describes polyurethane composites wherein special polyurethane covering layers for a fibre laminate impregnated with a 2-C PUR resin, which coats a core layer (e.g. a paper honeycomb) are used. The 2-C PUR resin for example consists of MDI and a mixture of polypropylene triols and diols from ethylene oxide propylene oxide copolymers.
In WO 2003/101719, polyurethane-based composites and the methods of production are described. These are 2-C polyurethane resins with defined viscosities and specific gel times.
2-C PUR systems are also discussed in: “Fiber reinforced polyurethane composites: shock tolerant components with particular emphasis on armor plating” (Ratcliffe, Colin P.; Crane, Roger M.; Santiago, Armando L., AMD (1995), 211 (Innovative Processing and Characterization of Composite Materials), 29-37.) and in Fiber-reinforced polyurethane composites. I. Process feasibility and morphology. (Ma, Chen Chi M.; Chen, Chin Hsing. International SAMPE Symposium and Exhibition (1992), 37 (Mater. Work. You 21st Century), 1062-74.)
Apart from the different binder basis, moisture-curing lacquers largely correspond to analogous 2-C systems both in their composition and also in their properties. In principle, the same solvents, pigments, fillers and auxiliary substances are used. Unlike 2-C lacquers, for stability reasons these systems tolerate no moisture whatsoever before their application. Also known are physically drying systems based on non-reactive PUR elastomers. These are high molecular weight, linear, thermoplastic urethanes from diols and diisocyanates, preferably MDI, TDI, HDI and IPDI. Such thermoplastic systems as a rule exhibit very high viscosities and hence also very high processing temperatures. This critically hinders their use for prepregs.
In the production of prepregs with fibre composites, the use of powders in reactive systems is more unusual and until now has been limited to a few use fields. Probably the most common process for applying a powder onto a fibre surface is the fluidized bed process (fluidized bed impregnation). By means of an upwardly directed flow, powder particles are converted to a state wherein they exhibit fluid-like properties. This process is used in EP 590 702. In this, the strands of individual fibre bundles are floated apart and coated with the powder in the fluidized bed. The powder here consists of a mixture of reactive and thermoplastic powder, in order thus to optimize the properties of the matrix. Finally, individual rovings (fibre bundles) are laid together and several layers compressed under a pressure of 16 bar for about 20 minutes. The temperatures vary between 250 and 350° C. However, in the fluidized bed process irregular coating often occurs, in particular if the strands are not pulled apart. Concerning this, in US 20040231598 a method is proposed which functions similarly to the fluidized bed process. In this, an air flow transports the particles to the substrate and a uniform deposition of the powder is effected through a specific configuration.
A further process is described in US 20050215148. There uniform distributions of the powder on the fibres are achieved with the device just mentioned. In this, the particle size ranges from 1 to 2000 μm. In several experiments, coating is effected from one or from two sides. Through the uniform application of the powder, laminates with no air inclusions are obtained after subsequent compression of the prepreg.
A further application, WO 2006/043019, describes the use of epoxy and amino-terminated resins in powder form. In this, the powders are mixed and applied onto the fibres. Next, the particles are sintered on. The particle size lies between 1 and 3000 μm, but preferably between 1 and 150 μm.
This restriction of the particle size to rather small diameters is also recommended in a study by the Michigan State University. The theory here is that particles with small diameters will more likely be able to penetrate into cavities between individual filaments than particles with larger diameters (S. Padaki, L. T. Drzal: a simulation study on the effects of particle size on the consolidation of polymer powder impregnated tapes, Department of Chemical Engineering, Michigan State University, Composites: Part A (1999), pp. 325-337).
Apart from the prepreg technology, reactive powder systems are also used in other standard processes, for example in winding technology [M. N. Ghasemi Nejhad, K. M. Ikeda: Design, manufacture and characterization of composites using on-line recycled thermoplastic powder impregnation of fibres and in-situ filament winding, Department of Mechanical Engineering, University of Hawaii at Manoa, Journal of Thermoplastic Composite Materials, Vol 11, pp. 533-572, November 1998] or in the pultrusion process. For the pultrusion process for example fibre strands (towpregs) are coated with the powder and firstly wound and stored as so-called towpregs. One possibility for their production is described in an article in the SAMPE Journal [R. E. Allred, S. P. Wesson, D. A. Babow: powder impregnation studies for high temperature towpregs, Adherent Technologies, SAMPE Journal, Vol. 40, No. 6, pp. 40-48, November/December 2004]. In a further study, such towpregs were pressed together by the pultrusion process and cured to give material components [N. C. Parasnis, K. Ramani, H. M. Borgaonkar: Ribbonizing of electrostatic powder spray impregnated thermoplastic tows by pultrusion, School of Mechanical Engineering, Purdue University, Composites, Part A, Applied science and manufacturing, Vol. 27, pp. 567-574, 1996]. Although the production of towpregs and subsequent compression in the pultrusion process had already been performed with duroplastic systems, to a large extent only thermoplastic systems have until now been used in this process.
The objective was to find a prepregs system which can be handled without difficulty, i.e. is non-toxic, and which does not have the high viscosities of thermoplastic polyurethane systems with the attendant difficulties of fibre saturation and fabric saturation, and which does not have the short processing times of 2C polyurethane systems. A further objective of this invention was therefore to find prepregs with polyurethane matrix material which can be produced by a simple process, wherein the main emphasis should be placed on the handling and storage life of the prepregs.